The present invention relates to a hammer designed for use in a print head of a dot matrix printer or the like and, more particularly, to a hammer structure which includes a heat dissipating means, displaceable with the hammer, which retain and protect the coil, and to the location of and structure for mounting a print wire thereto.
A dot matrix printer is an apparatus which prints a plurality of closely spaced dots at high speed at selected locations on a paper strip to form letters, numerals or other intelligible symbols thereon. The dots are formed by causing contact between the paper and an ink impregnated surface at the desired locations by selectively electromagnetically displacing elongated print wires mounted within the print head.
One type of conventional dot matrix print head consists of a plurality of selectively electrically energizable solenoids, each of which has a separate print wire extending therefrom. The impact ends of the print wires are retained in position with respect to the paper, and each other, by a wire bearing having a plurality of closely spaced openings therein arranged in a matrix array. Energization of a selected solenoid results in the print wire associated therewith being displaced, such that the impact end thereof causes contact between the paper and the ink impregnated surface to print a dot in the desired location.
However, such heads are bulky and massive, as well as complex in structure, and therefore, relatively expensive to manufacture and maintain. Since the solenoids each require a space much greater than the distance between the impact ends of the print wires connected thereto, complicated arrangements of the solenoids are required for a sufficient number of solenoids to be incorporated into the head to provide the required number of print wires. For this reason, the solenoids had to be arranged in groups or banks at different levels or in arcuate arrays. When arranged at different levels, each group of solenoids was provided with print wires of different length, depending upon how far the group was spaced from the wire bearing. When arranged in an arcuate array, the print wires were curved to various degrees, according to the placement of each solenoid.
Solenoids generate a significant amount of heat upon repeated actuation. Because the solenoid actuators are packed closely together, the heat generated by the solenoids builds up rapidly. Thus, provision had to be made to dissipate the heat generated by the solenoids to prevent the heat build-up from destroying the head. In order to accomplish this result, massive metallic heat dissipating elements or sinks were affixed to the head frame adjacent the exteriors of the solenoids. While the presence of the massive heat sinks substantially increased the bulk and weight of the head, the mass thereof did not interfere with the displacement of the print wires because the solenoids, and thus, the heat sinks mounted adjacent the exteriors thereof, remain stationary as the print wires are displaced.
In order to reduce the weight, bulk and cost of the print head, solenoid actuators have recently been replaced with extremely thin, coil carrying hammer type actuators. Hammers of this type are so thin that a plurality of closely spaced, parallelly situated hammers can be mounted between a single pair of stationary magnets. Each hammer comprises a thin, flexible planar frame portion having a recess therein into which a flat coil is received. The coil carrying portion is suspended from a support, in cantilever fashion, by an elongated flexible portion, such that it is situated in a non-varying magnetic field created between the magnets. The leads of the coil are connected to circuitry designed to electrically energize the coil when actuated. A print wire is mounted to and extends from the bottom of the frame portion and is displaceable therewith. When the coil is electrically energized, sufficient electromagnetic force is developed to displace the hammer from its original position such that the impact end of the print wire is moved to cause a dot to be imprinted on the paper.
Since each hammer must be extremely thin to permit a plurality thereof to be mounted in the small space between the magnets, the thickness of the coil and, thus, the number of wire turns in the coil, is limited. The strength of the permanent magnets is also limited, and thus, the amount of electromagnetic force developed by energization of the coil is relatively small. Moreover, the printer must operate at relatively high speeds and, thus, the response time of the hammer must be short. Therefore, the hammers must be designed to have the smallest possible mass and thickness, such that the space required therefor and the inertia thereof are minimal. With minimal inertia, even the relatively small amount of electromagnetic force developed will be sufficient to displace the hammer at the required high speed.
The flat coils mounted on the hammer frames also generate heat when electrically energized. Since the amount of space provided for each hammer is extremely small and the hammers are spaced closely together, a significant amount of heat build-up occurs during operation of this type of head also. However, this heat is difficult to dissipate in a manner which does not interfere with the operation of the head.
The flat coil is mounted on, and carried by, the displaceable hammer frame. To be effective, it is necessary that any heat dissipating device be mounted in thermal communication with the coil. Thus, the heat dissipating device must also be mounted on and displaceable with the hammer. However, conventional heat sinks inherently require a large amount of space and have a significant amount of mass. Such heat sinks cannot be used in this situation because the space required for, and the mass of, the heat sink would be far greater than the space allotted for, and the mass of, the hammer itself, thereby significantly increasing the space required for each hammer and the inertia of the hammer. Displacement of a hammer of such increased size and mass would require a much greater electromagnetic force than can be developed in this type of head.
Since the use of conventional heat dissipating devices is clearly contraindicated in this situation, a method of air cooling the hammer has been attempted. Openings in the top and bottom of the head have been provided, one of which is connected by means of a conduit or the like to an air blower or fan. The blower or fan continuously provides a stream of cool air through the head as the head is being operated. While an air cooling system such as this is capable of removing the heat generated by the hammers, it increases the size, weight and complexity of the printer, as well as generating additional noise and vibration. It is not, therefore, the optimum solution to the heat accumulation problem.
Another problem associated with hammers of this type relates to the structural strength of the hammer and, particularly, that part of the hammer where the flexible elongated portion, which serves to mount the hammer to the head, joins the frame portion, which carries the flat coil. This part of the hammer is the part most vulnerable to stress developed when the print wire impacts the paper and, thus, is most apt to fracture. While it would certainly be possible to structurally reinforce this part of the frame by making same thicker, as compared to the remainder of the frame, or embedding reinforcing elements therein, both of these solutions result in an increase in the mass and thickness of the hammer, both of which are to be avoided.
A third problem associated with hammers of this type relates to the manner of mounting the print wire thereto. Normally, the hammer frame is stamped out of a sheet of aluminum, because of the high strength per unit weight and flexibility of this substance. The print wire is normally composed of tungsten, a substance which is extremely wear-resistant. There is, however, no conventional method or structure known which can form a joint or bond between an aluminum element and a tungsten element with sufficient strength and rigidity to withstand forces of the magnitude to which the hammer will be subjected. Thus, various complex ways of making the joint between the hammer and the print wire have been attempted. However, none of these mounting methods has heretofore been acceptable.
It is, therefore, a prime object of the present invention to provide a hammer for a dot matrix print head or the like having heat dissipating means mounted on and displaceable with the hammer.
It is another object of the present invention to provide a hammer for use in a dot matrix print head or the like wherein the heat dissipating means comprises a thin sheet of thermally conductive metallic foil which is affixed to the hammer frame.
It is another object of the present invention to provide a hammer for use in a dot matrix print head or the like wherein the heat dissipating means comprises a pair of thermally conductive metallic foils, mounted on opposite sides of the frame, so as to retain the coil intact and in proper position with respect to the frame.
It is another object of the present invention to provide a hammer for use in a dot matrix print head or the like wherein the exterior surface of the thermally conductive metallic foil sheet acts as a bearing surface protecting the coil and frame from wear caused by contact with other parts of the head, as the hammer is displaced.
It is another object of the present invention to provide a hammer for use in a dot matrix print head or the like wherein the mounting structure for the print wire is located on the frame portion at the center of percussion of the hammer, so as to reduce torsional vibrations within the hammer normally caused by impact of the print wire and, thus, stress on the part of the hammer joining the flexible elongated mounting portion with the coil carrying portion.
It is another object of the present invention to provide a hammer for use in a dot matrix print head or the like wherein a novel structure for joining the tungsten print wire to the aluminum hammer is provided.
In accordance with the present invention, a hammer for use in a dot matrix print head is provided. The hammer carries an electrically energizable coil which is situated in a magnetic field and is displaceable relative to the field, between a rest position and a print position, when the coil is energized. The hammer comprises a coil carrying portion and means for resiliently mounting the coil carrying portion to the head. The coil carrying portion comprises a frame to which a flat coil is mounted and means, mounted on the frame and displaceable therewith, for dissipating heat from the coil.
The heat dissipating means comprises a sheet of thermally conductive foil affixed to the frame and extending therefrom into contact with the coil. The foil is extremely thin and has very small mass so as not to increase the size or inertia of the hammer.
The frame has a recess therein into which the coil is received. The thermally conductive foil sheet has a first portion affixed to one side of the frame and a second portion which extends from the frame over at least a part of the recess so as to contact and substantially cover a side of the coil.
The coil has a central opening therein. Preferably, the second portion of the sheet also has an opening therein. The coil opening substantially coincides with the opening in the second portion of the sheet. The outer periphery of the sheet substantially coincides with the outer periphery of the frame.
The sheet has a smooth exterior surface. The exterior surface forms a bearing surface, protecting the frame and the coil mounted thereto from wear caused by contact with other parts of the head, as the hammer is displaced.
The coil has two leads extending therefrom. The mounting means comprises a flexible elongated member, having a recess therein into which one or both of the leads is situated. The thermally conductive foil sheet preferably extends from the coil carrying portion, along the elongated member, so as to cover the recess and enclose the lead or leads therein.
Preferably, the heat dissipating means comprises first and second thermally conductive foil sheets, the sheets being affixed to different sides of the frame and respectively extending therefrom into contact with different sides of the coil. Thus, the coil is at least partially situated between the sheets so as to keep the coil intact and to accurately maintain the position of the coil relative to the frame. The exterior surface of each sheet comprises a smooth bearing surface which serves to protect the frame and the coil mounted thereon from wear caused by contact with other parts of the head as the hammer is displaced.
The hammer also comprises a print wire. A portion of the frame is provided for mounting the print wire. The print wire mounting portion is situated at the center of percussion of the hammer, so as to reduce the stress on the part of the hammer between the elongated portion and the coil carrying frame portion thereof. In addition, back-stop means are provided extending from the frame portion, at a position in alignment with the print wire mounting portion.
The print wire mounting portion of the frame comprises a part which extends from the coil carrying portion of the frame, the surfaces of which are substantially coplanar with the surfaces of the coil carrying portion and has print wire retaining means thereon. The print wire retaining means comprises a recess, elongated in the direction of hammer displacement, formed along the axis or center line of the mounting portion by cutting a series of slots therein so as to form a plurality of clamping elements which bridge or transverse the axis. The elements are bent such that alternate elements are located at opposite sides of the axis. The elements frictionally engage the print wire so as to join same to the frame. In addition, a thin layer of adhesive may be used to further secure the print wire.
Each element is compressed at the point where the element is adjacent the print wire, such that the combined thickness of the element and the radius of the print wire is approximately equal to one-half of the thickness of the hammer. Thus, the thickness of the portion of the frame which holds the print wire is substantially equal to the thickness of the remainder of the hammer. This print wire retaining structure results in a strong but relatively thin joint, because no additional thickness or mass is present due to the use of solder or other means used in prior art methods of attaching the print wire to the hammer.